The present invention relates generally to ballast circuits for fluorescent lights, and, particularly, to ballast circuits for a fluorescent lamp including a magnetic choke and an electronic starter circuit.
In the lighting of fluorescent lamps, a gas enclosed within a glass tube is caused to become ionized, thus reducing a breakdown voltage between electrodes placed at opposite ends of the glass tube. Ionization is initiated by heating of the electrodes. Once the gas is sufficiently ionized, a voltage at or above the breakdown voltage is placed across the lamp electrodes to thereby cause a current arc to form across the electrodes. The arc produces a bright glow within the lamp tube and produces radiation that activates a fluorescent coating on the inner surface of the glass tube, to thereby produce a bright light.
In controlling the turning on and off of fluorescent lamps, it is necessary to control the current to the lamp and to provide a starting voltage. In fluorescent lamps, this task is performed by a circuit called a ballast, also referred to as a ballasting circuit. There are generally two types of ballasts: magnetic ballasts and electronic ballasts.
Presently, most low wattage fluorescent lamps utilize magnetic ballasts that include magnetic chokes or suitable magnetic transformers and glow bulb starters. The magnetic choke limits current flow to the lamp while the glow bulb starter creates a voltage spike across the lamp after sufficiently preheating the electrodes. These magnetic ballasts are considered inefficient because of considerable power dissipation in the magnetic components. Moreover, these ballasts exhibit low power factors because of the highly inductive reactances of the magnetic chokes. The power factor is the ratio of the average (or active) power to the apparent power (root-mean-square [rms] voltages times rms current) of an alternating circuit.
Further, the glow bulbs associated with these ballasts exhibit random starting times that produce unpleasant flashes as an arc attempts to be established across the electrodes of the lamp. This is especially true at low line voltages because the ballasts permit too much voltage to be applied to the bulbs, due to the inadequacies in the ballast design. Arcs are then produced across the bimetal components of the bulbs as the voltage will be nearly high enough to sustain arcing, and annoying flickering and restriking occurs. As a result, the performances of glow bulbs are not predictable and this results in unreliable starting times of the fluorescent lamps.
Electronic ballasts are very expensive and can suffer from poor reliability due to the larger number of components involved. In these ballasts, a variety of electronic components are utilized to heat up the electrodes of the lamp and to establish the breakdown voltage across the electrodes. A most undesirable effect associated with these ballasts is the generation of annoying electromagnetic waves by the circuits due to high frequency typically greater than or equal to 20 KHz chopping of the alternating current power signal. These electromagnetic waves can interfere with the operation of appliances such as T.V.'s and radios.
Magnetic ballasts have reliability problems after 6,000 cycles because of contact wear-out in the associated glow bulb starters therewith. Electronic ballasts suffer from similar reliability problems because of the larger number of discrete components used.
U.S. Pat. No. 5,023,521 describes a hybrid ballast circuit for low wattage lamps. In this circuit, a magnetic choke is directly connected between a power supply and one electrode of the lamp to be lit. An electronic starter circuit is across the lamp to provide preheating and starting voltages. This circuit, however, enjoys only a moderate power factor of about 80%, for F13T5 type lamps, i.e., 13 watt lamps.